1. Field of the Invention
The present invention relates to a charging circuit, and in particular relates to a charging circuit for a bridge tied load (BTL).
2. Description of the Related Art
FIG. 1 is a schematic diagram of a conventional bridge tied load (BTL). When the resistances of resistors R1 and R2 are the same, if voltage Vcc is provided to a BTL 10 and a voice signal VAC is not provided to the BTL 10, a capacitor C1 starts charging. When the voltage of an endpoint 11 is equal to Vcc/2, the capacitor C1 stops charging. Therefore, the voltage of an endpoint 12 is approximately equal to the voltage of endpoint 11 and the voltage of an endpoint 13 is equal to Vcc/2 according to a characteristic of an amplifier.
Since a load RL is coupled between an endpoint 14 and an endpoint 15 coupled to ground, the voltage of endpoint 14 is equal to that of endpoint 15. When voltages of endpoints 13 and 14 are different, a capacitor C2 starts charging until the voltage of endpoint 14 is equal to that of endpoint 12 such as Vcc/2. When the voltage of endpoint 14 differs from that of endpoint 15, one current is generated across load RL. When load RL is a speaker, for example, the current generated across load RL causes the speaker to generate an abnormal voice.
FIG. 2a is a schematic diagram of another conventional BTL. When the resistances of resistors R1 and R2 are the same, at this moment, if voltage Vcc is provided to a BTL 20 and a voice signal VAC is not provided to the BTL 20, a capacitor C3 starts charging.
When the voltage of an endpoint 21 is less than Vcc/2, switch units SW1, SW2 are short. Therefore, the voltage of an endpoint 23 is equal to that of an endpoint 24 such that an abnormal voice is not generated by a speaker 27.
Due to the action of a capacitor C4, the period time that the voltage of endpoint 21 reaches to Vcc/2 is faster than that of an endpoint 22. For example, when the voltage of endpoint 21 is equal to Vcc/2 such as 6V, the voltage of point 22 is approximately equal to 5V at the same period time.
Since the voltage of endpoint 21 is equal to Vcc/2, switch units SW1 and SW2 are open. At the same time, the voltage of point 22 is approximately equal to 5V and the voltages of positive phase terminals of amplifiers 25 and 26 are approximately equal to Vcc/2, the voltage of endpoint 23 is approximately equal to 7V and that of endpoint 24 is approximately equal to 5V. Therefore, speaker 27 generates an abnormal voice. Additionally, when resistances of resistors R1 and R2 are the same, the voltage of endpoint 21 is equal to Vcc/2. However, when there is a fabrication difference that makes the resistances of resistors R1 and R2 different, the voltage of endpoint 21 is unequal Vcc/2. Such that switch units SW1 and SW2 are short and the BTL 20 is not operated.
FIG. 2b is a curve diagram of the voltage of endpoint 21 shown in FIG. 2a. Assuming that high voltage Vcc is equal to 5V. If the resistance of resistor R1 is equal to 50K and that of resistor R2 is equal to 49K, the voltage of endpoint 21 is approximately equal to 2.47V.
FIG. 3a is a schematic diagram of another conventional BTL. Capacitor C5 of this conventional BTL 200 is charged by a constant current source I such that the period time that the voltage of endpoint 201 reaches to Vcc/2 is faster than the voltage of endpoint 202. When voltages of endpoints 201 and 202 are different, voltages of endpoints 203 and 204 are different also. Therefore, speaker 207 generates an abnormal voice.
FIG. 3b is a curve diagram of the voltage of point 201 shown in FIG. 3a. Assuming that high voltage Vcc is equal to 5V. At time t1, the voltage of endpoint 201 is approximately equal to 2.5V.
FIG. 3c is a status diagram of speaker 207 shown in FIG. 3a. Although the voltage of endpoint 201 reaches 2.5V, the voltage of endpoint 202 is not equal to 2.5V at the same time. There is a voltage difference across speaker 207 approximately equal to 200 mV, so that the speaker 207 generates an abnormal voice.